In modern projection objectives for microlithography, a multiplicity of wavefront manipulators are used for correcting optical aberrations. Most of the manipulators bring about a wavefront correction via mechanical manipulation of the optical elements either via position change or via deformation or both in combination. As has been demonstrated, the manipulators have excellent correction properties for low-order wavefront aberrations such as typically occur when using the objectives in conjunction with so-called conventional settings and at a throughput of approximately 120 wafers/hour.
However, satisfying constantly increasing desired throughput properties involves ever higher light intensities in the objective and thus a constantly increasing thermal load acting on the optical elements. The thermal load causes wavefront aberrations, in the case of lenses predominantly by way of the temperature-dependent refractive index, in the case of mirrors predominantly as a result of the surface deformation on account of the thermal expansion. In addition there is the trend towards extreme illumination settings such as the dipole settings, for example, which entail a strong focusing of the optical power density on lenses near the pupil and can thus also cause higher-order and strongly localized wavefront aberrations. These can only be compensated for to a limited extent via the manipulators mentioned in the introduction. The same applies to wavefront aberrations caused by light-induced lifetime effects such as compaction which arise to an increased extent on account of the higher optical power densities. These wavefront aberrations, too, cannot be efficiently compensated for via the established manipulators. For this reason, these wavefront aberrations caused by lifetime effects are currently compensated for via exchangeable plates to which a specific correction asphere is applied. These compensation plates have to be exchanged repeatedly within the lifetime of the objective in order that the wavefront aberrations are kept small.